A Q&A session on the Rockstar Games Website addresses the frequent requests they've received for a PC edition of Red Dead Redemption, saying this is still not happening:

We hope that with the announcement that L.A. Noire is coming to PC this fall, and the knowledge that our next big game Max Payne 3 will have a PC release (plus that we’ve released both Grand Theft Auto IV and the Episodes from Liberty City for PC), we can finally put to rest any misconceptions that we’ve ‘abandoned the PC platform’. We do know that, yes, there is just one title absent from our PC release plans – that game of course being Red Dead Redemption, and of course we’re well aware that some fans have been asking for it. All we can say is that whenever it is viable (technically, developmentally and business-wise) for us to release a game for PC (or any other particular platform) – we will and we usually do; unfortunately, that is just not the case 100% of the time for all platforms. To PC gamers, we hope that between the Grand Theft Auto titles, and the forthcoming L.A. Noire and Max Payne 3, you’ll have plenty of great Rockstar action to enjoy.